Murine cytomegalovirus (MCMV) stimulates in mice NK cell activity 2 to 3 days after infection. Experiments are under way to determine whether this is entirely due to interferon. Spleen cells obtained after that time are refractory to stimulation by interferon. The question is whether this is due to the NK cell itself or a suppressor. Latent MCMV infection in mice can be activated by isoproteronol. Is this due to stimulation of cyclic nucleotides or due to increase in cellular DNA synthesis? Mouse macrophages elicitated by thioglycollate become permissive for MCMV. Infected macrophages are being studied for their antigenic properties in stimulating cytotoxic T cells in vitro, and for their ability to act as target cells in the assay for cytotoxic cells.